I did that a lot as a kid, where my parents would drop me off for two hours. I only did it once in recent years because I was single and terrified of missing that film at the movies.
I actually think it's the whole "being seen all by myself", mostly before the film or while everyone's getting seated. I wouldn't want them to think I have no friends. Although I always admire people who manage to just go and do it. I care too much what strangers think

But yes, it is a social thing, we usually combine it with eating out before or drinks after, so it's usually not just seeing a film.
My gf shares 98% movie taste with me, and is usually in, but I don't see her getting excited about Piranha 3D. I didn't talk with the guys about this one yet.